The invention relates to a decoding method for a CDMA [code-division multiple access] transmission system for demodulating a received signal available in serial code concatenation, a two-step coding being carried out at the transmitting end of the transmission system, including an orthogonal, multi-step modulation and an external error-correcting code of a predefined rate according to the definition of the species in claim 1, as well as to a device for implementiiod according to claim 13.
Transmission systems having an internal code, naamely orthogonal modulation using Walsh functions or rows of the Hadamard matrix as code words, and an external code, for example a hash [convolution] code with interleavers [interleave factors] are known in decoding methods. One simple concept entails decoding the internal code in accordance with the xe2x80x9cmaximum likelihoodxe2x80x9d principle, and decoding the external code using a Viterbi algorithm (soft-in, hard-decision out) (Proakis, J. G. Digital Communications, 2nd edition, New York, McGraw-Hill, 1989).
The next, best step involves improving the decoding of the internal code and in employing a xe2x80x9csymbol-by-symbolxe2x80x9d MAP decoder (the decoding of the external code remains unchanged thereby), the algorithms for this, both for a coherent receiving plan, as well as for an incoherent receiving plan, being generally known.
The German Patent 39 10 739 C3 describes a method for generalizing the Viterbi algorithm and, for this purpose, a device for receiving signals transmitted via an interfered channel, where, in a incremental metrics unit (TMU), the transitional costs are generated, and addition [summing], comparison, and selection [processes] are subsequently carried out, and the differential costs of two incoming paths are calculated.
In the Proc. IEEE International Conference on Communication (ICC), Geneva, Switzerland, pp. 1064-1070, May 1993, Berrou proposes decoding the parallel concatenated codes in an iterative [process].
The object of the invention is to improve upon the method according to the defined species, so as to enable an existing signal format to be decoded on the receiver side of a transmission system more efficiently and with less likelihood of bit errors than is possible with known methods heretofore, so that by using a downstream decoder circuit, the two-step, serially concatenated coding is decoded as optimally as possible and with the least possible outlay.
In the method according to the present invention, a soft-in/soft-out decoder is used in the receiver, at whose input and output, soft values are processed as reliability information (L values), the soft output of the first decoder step (internal code) being the soft input for the decoder step (external code) that follows in each case, and the first decoder step receiving the output values of the preceding demodulation, which contains the channel""s reliability information.
Thus, the methods of iterative decoding can advantageously be applied to a CDMA system having orthogonal modulation as an internal code and a hash code (inclusive of interleavers) as an external code, thus within a system having serial code concatenation.
At the core of iterative decoding is the decoding instruction for the internal code. In this context, the decoding instructions must be expanded to enable use of a-priori information about the symbols to be decoded. Moreover, the external xe2x80x9chard decisionxe2x80x9d Viterbi decoder is replaced by a xe2x80x9csoft decisionxe2x80x9d Viterbi algorithm, ieby a MAP algorithm having a xe2x80x9csoft decisionxe2x80x9d output for the coded bits, to obtain the a-priori information for the renewed decoding.
In another embodiment of the method, a soft output is used by a decoder, in particular a MAP decoder, as a-priori information for the systematic bits of the Walsh function of the internal code for decoding of the same. To enhance the reliability of the decisions of the internal decoder, a feedback (iterative decoding) from the external to the internal decoder can be performed at least once, and the decisions (extrinsic information) of the second, external decoder about the systematic bits of the code words of the internal code, for example of the Walsh functions, can be fed back as a-priori information to the input of the first, internal decoder.
The a-priori information for the systematic bits of the code words of the internal code, for example of the Walsh functions, are likewise made available to the internal MAP decoder (maximally a posteriori) as reliability values in vector L(u), and the decoder supplies, as a result, the L-values for estimated symbols L(û), the amount |L(ûk)| of the L-values indicating the reliability of the decision and the operational sign(L(ûk) of the L-values representing the hard decision. In the coherent receiver structure, the internal MAP decoder calculates, starting from the input vector (Lcxc2x7y) having a specific reliability (Lc) and [from] the a-priori vector L(u), as a decoder result, the weighted decision (L-values, L(ûk)) for the estimated symbols, as well as the extrinsic term (Le(ûk)) of the L-values.
In the coherent receiver structure, to decode the internal Hadamard code, the vector of a-priori values (L(u)) for the systematic bits is added to the vector of reliability values (Lcxc2x7y) from the channel and, after that, a fast Hadamard transformation (FHT) follows; subsequently, with the signals (vector w), the exponential functions are generated with xc2xdxc2x7wj as an argument, after which, the elements of the result vector (z) for each symbol (ûK) to be decoded are added, divided and expressed logarithmically, according to the equation:                               ln          ⁢                      xe2x80x83                    ⁢                                                    ∑                                  j                  ,                                                            u                      k                                        =                                          +                      1                                                                                        N                  -                  1                                            ⁢                              z                j                                                                    ∑                                  j                  ,                                                            u                      k                                        =                                          -                      1                                                                                        N                  -                  1                                            ⁢                              z                j                                                    =                  xe2x80x83                ⁢                  ln          ⁢                      xe2x80x83                    ⁢                                                    ∑                                  j                  ,                                                            u                      k                                        =                                          +                      1                                                                                        N                  -                  1                                            ⁢                              exp                ⁡                                  (                                                            1                      2                                        ⁢                                          xe2x80x83                                        ⁢                                          w                      j                                                        )                                                                                    ∑                                  j                  ,                                                            u                      k                                        =                                          -                      1                                                                                        N                  -                  1                                            ⁢                              exp                ⁡                                  (                                                            1                      2                                        ⁢                                          xe2x80x83                                        ⁢                                          w                      j                                                        )                                                                                            =                  xe2x80x83                ⁢                              ln            ⁢                          xe2x80x83                        ⁢                                          (                                                      ∑                                          j                      ,                                                                        u                          k                                                =                                                  +                          1                                                                                                            N                      -                      1                                                        ⁢                                      exp                    ⁡                                          (                                                                        1                          2                                                ⁢                                                  xe2x80x83                                                ⁢                                                  w                          j                                                                    )                                                                      )                                            Term                ⁢                                  xe2x80x83                                ⁢                1                                              -                                    ln              ⁢                              xe2x80x83                            ⁢                              (                                                      ∑                                          j                      ,                                                                        u                          k                                                =                                                  -                          1                                                                                                            N                      -                      1                                                        ⁢                                      exp                    ⁡                                          (                                                                        1                          2                                                ⁢                                                  xe2x80x83                                                ⁢                                                  w                          j                                                                    )                                                                      )                                                    Term              ⁢                              xe2x80x83                            ⁢              2                                                                        xe2x80x83                ⁢                  ≈                      xe2x80x83                    ⁢                                    ln              ⁢                              xe2x80x83                            ⁢                              (                                                      max                                          j                      ,                                                                        u                          k                                                =                                                  +                          1                                                                                                      ⁢                                      [                                          exp                      ⁡                                              (                                                                              1                            2                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      w                            j                                                                          )                                                              ]                                                  )                                      -                          ln              ⁢                              xe2x80x83                            ⁢                              (                                                      max                                          j                      ,                                                                        u                          k                                                =                                                  -                          1                                                                                                      ⁢                                      [                                          exp                      ⁡                                              (                                                                              1                            2                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      w                            j                                                                          )                                                              ]                                                  )                                                                            =                  xe2x80x83                ⁢                                            1              2                        ⁢                                          max                                  j                  ,                                                            u                      k                                        =                                          +                      1                                                                                  ⁢                              [                                  w                  j                                ]                                              -                                    1              2                        ⁢                                          max                                  j                  ,                                                            u                      k                                        =                                          -                      1                                                                                  ⁢                                                [                                      w                    j                                    ]                                .                                                        
The decoder result for bit (ûhd k) is made up of three terms, namely of the a-priori information [L(uk)] about the bit to be decoded, of the channel information [Lcxc2x7ysys(k)] about the bit to be decoded, as well as of the extrinsic information [Le(ûk)], in which the channel and a-priori information of all other bits of vector (y) or of the transmitted Walsh function are combined.
In the incoherent receiver structure, the internal MAP decoder calculates, starting from the input vector (w) and the a-priori vector (L(u)), as a decoder result, the weighted decision (L-values, L(ûk) for the estimated symbols, as well as the extrinsic term (Le(ûk) of the L-values.
In the incoherent receiver, to decode the internal Hadamard codes, the a-priori information (L(u)), for example in the form of a-priori probabilities P(xj) for the Walsh functions, enters into the decoder instruction, which is such that for each bit decision, multiplied into the summing of numerator or denominator of the term to be expressed logarithmically of each considered element of the decision vector (w), are three terms, since there are the a-priori probabilities P(xj), the exponentiated vector element, as well as the modified Bessel function to the first type of power (Lxe2x88x921) with argument.
A device for implementing the method is characterized by a soft-in/soft-out decoder in the receiver, at whose input and output, soft values are processed as reliability information (L-values), the soft output of the first decoder step (internal code) being in each case the soft input for the subsequent decoder step (external code), and the first decoder step receiving the output values of the preceding demodulation, which contains the reliability information of the channel. To enhance the reliability of the decisions of the internal decoder, a feedback (iterative decoding) from the external to the internal decoder is performed at least once, the decisions (extrinsic information) of the second, external decoder about the systematic bits of the code words of the internal code, for example of the Walsh functions, being fed back as a-priori information to the input of the first, internal decoder. In the process, a RAKE receiver can be arranged before [upstream from] the first decoder step, which receives the output values of the preceding demodulation.